In a recent report (Hallgren et al., J. Biol. Chem. 252: 1014-1022, 1023-1033, 1034-1040, 1977) described the isolation and chemical structure of several oligosaccharides in urine from 3 pregnant or lactating women. Some of these were oligosaccharides with women Le a or Le b blood group activity. Using a radioimmunoassay developed in our laboratory we examined the urinary excretion of Le b-active oligosaccharides in urine of 138 pregnant or lactating women of different Lewis blood types. The amount of Le b-active oligosaccharides in urine during pregnancy and lactation correlates with Lewis blood type and with the strength of expression and Le b antigen on erythrocytes. The site of origin oligosaccharides has not been defined but several lines of indirect evidence suggests they may originate in mammary gland.